It's called Google Latitude, but it doesn't trace phone numbers; instead, it tells you where your friend(s) are at the time they call you. Tracing a phone call can only be done by your telephone company.

If you're using a cell phone, go to google.com/latitude to download Google Maps with Latitude; if you're using your computer, go to http://www.google.com/latitude and add the Latitude app to your iGoogle homepage.